Photodynamic diagnosis (PDD) and photodynamic therapy (PDT) have been used in the medical field. PDD is a diagnosis utilizing characteristics such that a light-sensitive substance that generates fluorescent light by the irradiation of excitation light is specifically accumulated in tumor tissues wherein the light-sensitive substance is administered in a living body in advance and the tumor tissues are observed based on the fluorescent light emitted from the substance. PDT is a therapy that destroys tumor tissues utilizing singlet oxygen generated by the excitation of a light-sensitive substance.
With PDD or PDT, it is required to be able to photograph and observe fluorescent light emitted from the light-sensitive substance in tumor tissues with a high degree of accuracy under strong illumination light such as shadowless light in an operation room. However, the fluorescent light emitted from the light-sensitive substance is extremely weak as compared with the illumination light, thereby causing such a problem that a fluorescent light image is lost in the subject image formed by the illumination light.
On the other hand, in recent years, there has been a widespread attention to a diagnosis method including administering indocyanine green (ICG) in a living body as a optical contrast agent, exciting ICG by the irradiation of excitation light or the like, and capturing the image of near-infrared fluorescent light emitted by ICG along with the subject image to observe these images. Hemoglobin has an absorption in a short wavelength of less than 600 nm, and water has an absorption in a long wavelength of more than 900 nm. On the other hand, the excitation wavelength of ICG and the wavelength of fluorescent light emitted by ICG are each in a wavelength band of 600 to 900 nm in which hemoglobin or water has no absorption. Therefore, by using ICG, it becomes possible to observe the inside of a living body. With this method, however, there is a problem that the fluorescent light image is lost in the subject image.
In order to solve such a problem, it has been proposed that when a subject image containing fluorescent light in an infrared region is separated into RGB components and the RGB components are superimposed on one another to form a color image, the separation is performed so that the R component is enhanced, thereby emphasizing the observed area of weak fluorescent light (Patent Document 1). With this method, however, since not only fluorescent light in the infrared region but also light of the R component forming the subject image are emphasized simultaneously, it becomes difficult to accurately observe the area emitting fluorescent light in the subject image. Moreover, there is another problem such that it is required to provide a filter for separating the subject image into RGB components and a filter driving mechanism, thereby complicating the device configuration.
Moreover, it has been proposed that an observed image formed by weak fluorescent light emitted by an observed area in a subject and a subject image formed by the reflected light of the subject are simultaneously observed when the subject is illuminated by means of an illuminator containing excitation light and illumination light, wherein the intensity of an excitation light component and the intensity of an illumination light component are adjusted, so that the brightness values or contrast values of the observed image and the subject image are controlled (Patent Document 2). However, in the case where such an illuminator is used, not for endoscopic observation, but for observing tissues exposed to the strong illumination light in an operation room or the like, it is required to substantially reduce the illumination light component, thereby causing a problem such that the operation room must be darkened.
Furthermore, it has been proposed an imaging device for simultaneously photographing an observed image formed by fluorescent light in the infrared light band and a subject image in the visible light band, wherein an optical filter for cutting excitation light and transmitting therethrough infrared light and visible light is used, and wherein light transmittances are made different from each other between infrared light and visible light, so that a suitable balance between the infrared light image and the visible light image is achieved (Patent Document 3). However, fluorescent light in the infrared light band forming the observed image is extremely weak as compared with illumination light in the visible light band forming the subject image. Therefore, it is difficult to obtain an optical filter such that the balance between the transmittance in the infrared light band and that in the visible light band is well adjusted. Furthermore, since the focal plane of fluorescent light in the infrared light band forming an observed image is different from the focal plane of light in the visible light band forming a subject image, it is not possible to simultaneously and clearly capture the observed image and the subject image.
As opposed to this, it has been proposed to use, in a fluorescent light endoscopic device, a filter stop that is concentrically divided into three sections. The innermost section thereof is a circular visible light transmitting section, the outer side of the innermost section is a visible light non-transmitting section, and the outermost section is a light shielding section. By using such a filter stop, a fluorescent light image is lightened up with respect to a visible light image, thereby making it possible to observe the visible light image at a deeper depth of focus (Patent Document 4). However, when this filter stop is used for observing tissues exposed to illumination light in an operation room or the like, the intensity of illumination light greatly varies depending on specific observation conditions. Thus, it is not possible to suitably adjust the brightness of the fluorescent light image with respect to the visible light image.